Prior art switching power sources of flyback pseudo resonance (partial voltage resonance) type have been utilized in wide applications because they advantageously have a simplified circuit structure with the high conversion efficiency and less operating noise. For example, Japanese Patent Disclosure No. 2003-61345 discloses a switching power source of such a pseudo resonance type which, however, has their defect in that change in input voltage from commercial AC power source fluctuates switching frequency of a switching element under the constant load. For example, in a switching power source applicable to power source voltages all over the world with 100 watts of rating output power and 85 to 264 volts of commercial AC input voltage, the switching element is operated with 50 kHz of switching frequency under the commercial AC input voltage of 85 volts, however, it increases the switching frequency up to 100 kHz with 264 volts of commercial AC input voltage.
In such a switching power source, as the on period of the switching element is shortened with elevation of power voltage applied on a primary winding of transformer, power voltage supply in a high range (185 to 264 volts) increases the switching frequency and thereby switching number of the switching element. Accordingly, a problem arises that the power source gives rise to increase in switching loss and deterioration in conversion efficiency under the power voltage supply in the high range.
Japanese Patent Disclosure No. 2001-231257 exhibits a DC-DC converter capable of changing either pseudo resonance or frequency reduction by switching minimum off period of the switching element based on load condition on the secondary side. This DC-DC converter controls increase in switching number of switching element per time under the light load condition by shifting to pseudo resonance under the heavy load condition and shifting to frequency reduction under the light load condition. However, the power supply of high voltage range increases switching frequency and switching number per time of switching element for the above-mentioned reason, causing increase in switching loss and deterioration in conversion efficiency in this DC-DC converter. Also, in the transition from pseudo resonance to frequency reduction or vice versa, a peak value of electric current flowing through transformer changes in a wide range depending on discontinuously varying oscillation frequency of transformer. Accordingly, under some load conditions, vibration of transformer undesirably produces acoustic sound or noise.
An object of the present invention is to provide a switching power source which improves the conversion efficiency under the power voltage supply of wide range. Another object of the present invention is to provide a switching power source capable of controlling noise resulted from transformer etc.